Coating metallic sheet or strip material with powdered annealing separator substances



Sept. 19, 1961 J. M. JACKSON ETAL 3,000,752

COATING METALLIC SHEET OR STRIP MATERIAL WITH POWDERED ANNEALING SEg%RATgR7SUBSTANCES Filed Dec irw $55 m a 0 I Tn m a H 1 VK Y m A JM v United States Patent 3 000 7 52 COATING METALLIC SIiEET OR STRIP MATE- RIAL WITH POWDERED ANNEALING SEPARA- TOR SUBSTANCES John M. Jackson, Middletown, and Philip A. Macklin, Ox-

ford, Ohio, assignors to Armco Steel Corporation, Middletown, Ohio, a corporation of Ohio Filed Dec. 30, 1957, Ser. No. 706,090 12 Claims. (Cl. 117-17) The invention relates to the preparation of metallic sheet or strip materials for heat treatments necessitating the use of an annealing separator. Annealing separators are generally materials of a refractory character employed in high temperature annealing operations to prevent stacked metallic sheets or the convolutions of a coiled strip from sticking together by welding during high temperature heat treatments. In some instances, annealing separators have other uses in addition. For example, as set forth in United States Patent No. 2,385,332, in the names of Victor W. Carpenter et al., portions of an annealing separator of magnesium oxide can, during a heat treatment at suitable temperatures, be caused to fuse with silica particles on or near the surfaces of silicon-iron sheet stock to form a glass-like coating, which coating is useful as an interlamination insulator in the use of silicon-iron in electrical apparatus, e.g. in the cores of transformers. Herein the term annealing separator will be employed in a sense broad enough to cover materials which have functions in addition to the bare function of separation.

Annealing separators may include a wide variety of substances such as calcium oxide, alumina, silica, or other refractory oxides, lime, and the like; but for purposes of an exemplary disclosure herein, the invention will be described in connection with the use of magnesia, e.g. MgO, it being understood that the invention is not so limited.

Magnesia has hitherto been in widespread use as an annealing separator in the production of iron and steel sheet stock and in particular for the production of siliconiron sheets and strips in annealed condition. The usual process of applying the magnesia involves slurrying it in water, applying the slurry to the surfaces of the sheets or strips, and metering the coating thereon by suitable means such as rubber covered rollers. When the water vehicle of the slurry has been evaporated, reasonable adhesion of the separator is attained. There are various grades of magnesia, some of which hydrate more rapidly than others; but whenever magnesia is slurried with water, some portion of the magnesia is hydrated to form magnesium hydroxide. Before a high temperature heat treatment can be accomplished, it is generally necessary to subject the coated sheets or strip to a preliminary heat treatment in which the water of constitution of the magnesium hydroxide is driven off. Such pretreatments are not only expensive in themselves, but the driving off of the water affects the nature of the atmosphere in the furnace by giving it a high dew point; and the result is frequently an oxidation of the sheet or strip stock, sometimes resulting in brittleness. The heat treatment necessary for drying is different from the heat treatment in which the magnesia is to serve as an annealing separator, especially since it is usually necessary to change atmospheres between the two heat treatments so as to get rid of the evolved moisture.

There has long been a need for an apparatus and method whereby sheet and strip stock may be coated with an annealing separator in dry condition and devoid of water of constitution which will be evolved during the heat treatment. It is a primary object of this invention to provide such a means and method.

In coating sheet or strip stock with a slurry of re- "ice fractory material and a vehicle, it is diflicult to obtain complete uniformity of coating. Hence, it is generally necessary to use somewhat more of the coating substance than would be required on a theoretical basis to provide a coating of a given or selected thickness. Also, it is difiicult, when coating with a slurry, to provide against the occurrence of uncoated areas or areas having less than the desired thickness of coating. It is an object of the invention to provide a means and method for the more uniform coating of sheet or strip stock with a powdered annealing separator.

Where there is applied to strip or sheet stock a coating of annealing separator such that not only must free water be driven out but also water of constitution eliminated, and where the sheets are stacked or the strip material coiled preparatory to being given the heat treatment, nonuniformity of surface conditions in the material being treated is frequently encountered because of specific differences in temperature between different parts of the material and because of time delays in the diffusion of atmospheres between stacked sheets or the convolutions of a coil. Thus, edges of the material are likely to be in a different condition from portions thereof substantially spaced from the edges. It is an object of the present invention to provide a means and a method for coating sheet or strip stock with an annealing separator in such a manner that local differences in atmosphere in a furnace, in which the material is being treated in a stacked or coiled condition, can be avoided.

These and other objects of the invention, which will be set forth hereinafter or will be apparent to one skilled in the art upon reading these specifications, are accomplished by that construction and arrangement of parts and by that procedure of which an exemplary embodiment will now be described. In these specifications, there is set forth in detail the best mode known for accomplishing the purposes of the invention, taking up each step in detail, and indicating with respect to such steps those variations from the preferred practice as have been found acceptable. Reference is made to the accompanying drawings wherein:

FIG. 1 is a diagrammatic representation of a strip of material showing the location of various electrodes with respect thereto, and

FIG. 2 is an elevational representation of specific apparatus which has been employed in the practice of the invention.

Attempts have been made hitherto to deposit magnesium oxide or other annealing separators devoid of water of constitution and in dry form on sheet or strip materials. These attempts have not been successful for, while mechanical means can be arranged to give a reasonable uniformity of distribution, there is no substantial adhesion of the particles of the separator either to the sheet or strip materials or to each other. This means that the coated materials cannot be handled in the usual fashion. It is not possible to dust dry separator particles on sheets and then stack the sheets, or to dust the particles on strip stock and then coil the stock, while maintaining desired uniformity and continuity of the coating. Adhesive substances which could be added to the separator or applied to the sheet or strip material, and which would not interfere in some way either with the subsequent heat treatment or with the qualities of the stock, have not been discovered.

It has been realized that particulate substances could be deposited on surfaces by means of an electrostatic field; but so far as is known, such procedures have been followed only where adhesion was of no consequence, or where the particles were themselves of an adhesive character, or where the surfaces were precoated with adhesive in which the particles became embedded, or where the particles were of such nature that they could be rendered adhesive or penetrating immediately after coating, as by melting under a heat treatment.

It has, however, been found in the practice of this invention that magnesium oxide or other annealing separator can be deposited upon the surface or surfaces of metallic sheet or strip material by electrostatic means not only with complete uniformity of coating but also with a surprising adhesion such as will permit the stacking of sheets or the formation of a convoluted coil, which then may be handled in the usual fashion in accordance with steel mill practice, without dislodgment of the coating, under the particular This means that sheets, coated as hereinafter taught, may be sent through a continuous annealing furnace, or if stacked, may be handled as a stack and transferred to box annealing apparatus without dislodgment of the coating, and that strip similarly coated, may be coiled and transferred to box annealing equipment by ordinary coil handling equipment, again without dislodgment of the coating. In fact, the coating is resistant against dislodgment excepting where a forcible scraping action is encountered.

In the practice of the invention, the annealing separator, MgO, in the exemplary embodiment, in dry form and devoid of water either as a vehicle or as water of constitution, is metered from a suitable source to a point at which it can be picked up by a moving stream of air or other gas, through the action of a Venturi device. The stream of gas containing the suspended magnesia passes through an air feed channel and enters a space between a metallic strip or series of sheets to be coated and a plurality of electrodes in the form of wires or rods extending transversely the direction of movement of the sheets or strip. The material to be coated is preferably grounded and a high voltage positive electrodes. place about plained; but

A positive corona discharge is caused to take the electrodes, as will hereinafter be excorona discharge at the sheet or strip must be avoided. The gas surrounding the electrodes becomes ionized, and in a region adjacent the strip or sheet, there will be a predominance of positively charged ions. When the magnesia is carried into the last mentioned region, its particles become positively charged; and the electrical field then acts upon the charged particles to propel them toward the sheet or strip. Under proper conditions, it is thought that the magnesia particles lose their charge upon contact with the strip or sheet, but are held to its surface by molecular adhesion to neighboring particles and to the strip itself.

It is usual in the practice of the invention to pass the strip thereafter beneath an electrode in the form of a plate, the purpose here being to increase the evenness of the deposition or coating and to deposit the last remaining particles entrained in the gas, although in many instances the use of the plate electrode may be omitted.

There are a number of problems involved in the electrostatic deposition of an annealing separator on metallic strip or sheets with which this invention is particularly concerned. Some of these problems are:

(1) To secure a uniform deposition of the magnesia across the strip,

(2) To insure the deposition of a sufllcient quantity of the magnesia for the purpose desired,

(3) To avoid the deposition of magnesia upon parts of the apparatus other than the strip or sheets to be coated,

(4) To avoid supplying magnesia in such quantities that the strip or coil will slide when wound upon itself, and

(5) To insure that all of the magnesia, which is introduced into the ionization region, will be deposited upon the sheets or strip.

In connection with the last mentioned problem, while it is usual to provide a housing around the ionization and deposition areas, there will be a substantial movement of gas through such a housing, and it is entirely possible and highly desirable to have the gas issuing from the far end potential is applied to the conditions hereinafter outlinedn Venturi feed of the housing sufliciently free of entrained materials to permit its venting to the atmosphere of the work space, particularly where air is used as the entraining gas. Without departing from the spirit of the invention, however, means may be provided for the recirculation of gases exiting from the housing and the removal or re-utilization of any magnesia entrained therein. But if substantial quantities of magnesia are carried entirely through the housing by the moving gas stream, the efliciency of the process will be impaired.

Referring now to FIG. 2, the index numeral 1 represents a hopper containing a supply of the magnesia in finely divided form. The fineness of subdivision of the magnesia is not critical, so long as it is fine enough to be air-borne as hereinafter set forth. Without limitation, the magnesia may be such as to pass a screen of 325 meshes to the inch, or may have a particle size of about 44 microns.

The hopper may be provided with agitating means or vibrating means to prevent the bridging of the material therein, all as is well understood in the art. It is also provided with a means for feeding the magnesia in metered quantity, which feeding means should be adjustable. The feeding means may take various forms. In actual practice, a screen 2 for the magnesia has been incorporated in the hopper, and a motorized scraper or agitator 3 in connection with the screen, can be caused to meter the magnesia, since the amount fed will depend on the speed of the scraper blades.

The powdered annealing separator, which is fed from the hopper through delivery means 4, is entrained in a moving gas stream provided at conduit 5 by means of a powder injector system 6 of known form. It will be understood that a Venturi system will work to optimum advantage at some particular air injection speed. Hence, the desired speed of the moving gas stream must be taken into account in connection with the quantity of magnesia which is to be fed for coating purposes. A valve has been shown diagrammatically at 5a in conduit 5. The gas stream in which the magnesia is entrained then passes through a gas feed channel 7, which insures thorough mixing and dissemination of the magnesia in the moving gas and facilitates the delivery of the separatorladen gases to the point or region of ionization in the apparatus without the deposition of the magnesia on parts other than the strip or sheets.

Air (preferably dried air) may be used as the entraining gas and will normally be found highly satisfactory. However, reducing or neutral gases, such as hydrogen, gases rich in hydrogen, argon and the like, may be employed if desired. It has been found that where the separatorladen gas exits from the gas feed channel 7, it may to advantage be isolated from the electrodes by an envelope of gases not containing magnesia and delivered to the end of the air feed channel through additional conduits 8. This envelope of air or gas should -be adjusted to move at the same speed as the separator-laden gases passing through the air feed channel. With proper speeds of the entraining gas, there will not be any substantial buildup of magnesia on the walls of the air feed channel.

The strip to be coated is indicated at 9. It is withdrawn from a decoiler 10 and coiled again by a motorized device 11. Throughout the area where coating is to occur, the strip may be supported under tension by rollers indicated at 12 and 13. These rollers, if of metal, may be electrically grounded, as may also the decoiling and recoiling devices. The coating area is provided with a housing 14. This housing, if desired, can be made of transparent plastic so that the operations going on inside it can be observed. It made of'suitable plastic or other insulative substance, it can be employed to support the electrode structures next to be described.

The electrode arrangement is illustrated diagrammatically in FIG. 1. The strip is again indicated at 9. Rodlike or wire electrodes extending transversely to the strip are indicated at 15, 16, 17 and 18. These electrodes are connected to the positive side of a high voltage electrical supply diagrammatically indicated at 19 in FIG. 2. The number of electrodes of this type is exemplary, and may be varied. The area between the electrodes and the strip is designated as an ionization region.

It has been indicated that there should be a positive corona discharge from the electrodes 15 to 18 inclusive.

This corona discharge is such that it can be observed in subdued light as a purplish glow. The positive corona discharge from the electrodes 15 to 18 should be even and uniform, which, in turn, means that the electrodes themselves must be kept free and clean of magnesia or other foreign materials and that sharp points in the high voltage wiring must be avoided to prevent corona discharge there. While a positive corona discharge takes place at the electrodes 15 to 18, care must be taken to avoid a negative corona discharge at the strip or sheet material, since the result of such discharge is not only to charge some of the magnesia particles negatively and to cause them to deposit upon the electrodes instead of the strip, but also to interfere with the adhesion of the de posited coating. A negative corona is most likely to occur at the edges of the strip. To avoid it, the electrostatic field gradient is reduced at the strip edges by making the eifective or exposed portions of the electrodes 15 to 18 less in length (d3) than the width of the strip, by varying the effective or exposed length of the electrodes inversely to the electrode-to-strip distance (d2), and by varying the electrode-tolectrode spacing (d1). The distance between the electrodes and the strip (d2) should not, however, exceed about half the distance between electrodes (d1).

It has been noted that a negative corona also tends to occur when the coated strip is stopped or left stationary under the corona electrodes. Presumably, this is due to the formation of sharp powder points on the coated strip, due to excessive coating.

Magnesia or other foreign materials clinging to the electrodes 15 to 18 not only tends to produce a non-uniform corona, but may initiate a continuous discharge between the electrodes and the strip. A negative corona occurring at the strip tends also to initiate a continuous discharge. A continuous discharge tends to charge a substantial part of the magnesia powder negatively, with consequent fouling of the electrodes.

In actual operation the air feed channel is so disposed that the magnesia does not enter the immediate vicinity of the electrodes. Thus, negative ions and electrons formed by the positive corona discharge at the electrodes pass immediately to the electrodes themselves, while all of the magnesia becomes positively charged and migrates to the strip.

It will be understood that vary with different one such operation,

the operating conditions will arrangements and sizes of parts. In using a .008 inch diameter tungsten wire, a potential of from 28 to 31 kilovolts was applied between the electrodes 15 to 18 and the strip. When the apparatus was operating properly, it was observed that the corona current did not exceed l95 microamperes maximum. A corona current of about 100 to 150 microamperes is preferred. In the particular operation the air speed was 600 ft. per minute at the inlet to the Venturi device and 400 ft. per minute at the end of the air feed channel, i.e at the entrance to the electrostatic chamber.

To choose an operating voltage, a simple test method is to apply an increasing potential to the electrodes, observing the point at which arc-over or negative corona occurs, and then cut back the voltage by an amount of 2 to 4 kilovolts for operating purposes.

Following the transverse electrodes 15 to 18 inclusive, there is an electrode 20 preferably in the form of a plate of brass or other suitable metal. This plate is connected to its own voltage supply or to a tap on the first mentioned voltage supply so that the potential applied to it will be about one-half the potential applied to the transverse electrodes. This normally avoids negative corona on the strip or sheet material beneath the plate electrode. The function of the plate electrode is to insure the deposition of any magnesia remaining in the air or other gas passing through the housing, and also to promote an evenness of distribution of the magnesia which has already been coated onto the strip.

It may be found desirable to include a series resistance in the high tension lead to the electrodes 15 to 18. A 20-megohm resistor has been employed in this lead and is believed to be useful in preventing or quenching arcovers.

By the use of the process hereinabove described, it has been found readily possible to apply a coating of .06 oz. per square foot of surface of magnesia to the strip. This figure is chosen for illustrative purposes because it is comparable in weight of magnesia to coatings hitherto applied by the water-slurry method. However, coatings applied by the method of this invention are more uniform and even than coatings otherwise applied and hence can be thinner. Consequently, the figure of .06 oz. per square foot should be regarded as illustrative merely and not as limiting. At the same time it will be understood that there is no benefit in an unusually thick coating of magnesia or other annealing separator where the coating can be made even and uniform. It will be found further that with any given annealing separator in any given state of subdivision, there will be an optimum quantity of it for entrainment in the air or other gas passing through the apparatus at a given velocity. If too little magnesia is entrained in the gas, it will be obvious that a greater quantity of gas will be required to be passed through the housing in order to obtain the desired coating at a given strip speed. This increases turbulence and makes it more difiicult to keep the magnesia away from the immediate vicinity of the electrodes. On the other hand, if too much magnesia is entrained in the gas, a condition may be encountered in which not all of the particles can become ionized to be deposited on the strip within the length of the coating apparatus. This makes for carryout at the end of the apparatus and is undesirable. Within reasonable limits, the thickness of the coating can be controlled by the speed of the strip.

The physical adherence of the powder to the strip or sheet stock is found to be unexpectedly good under the conditions herein described. There is, however, a limit to the thickness of coating which may be deposited without decreasing physical adherence. It is thought that when the coating becomes too thick, there is a retention of charge by at least the upper layers of the deposited powder, which prevents the molecular adhesion referred to above. While thicker coatings can be made under some circumstances, it has been ascertained that difficulty with adhesion due to coating thickness will not be encountered in coatings of 0.10 oz. per square foot or less.

Best results appear to be obtained when the deposition of the annealing separator occurs substantially completely in that area indicated in FIG. 1 as the ionization region.

While there has been described herein a means and method for coating the top side of a strip or succession of metallic sheets, it will be understood by the skilled worker in the art that the bottom side of the strip or sheets may be similarly coated either simultaneously or sequentially, since the deposition of the magnesia can occur upwardly as well as downwardly under the conditions hereinabove described. Thus a similar arrangement of electrodes may be made in the bottom of the housing 14, together with appropriate means for delivering the separator into the ionization region.

However, for many uses, coating one side of the strip or sheet material only will be found entirely adequate. This is especially true where the magnesia or other material is being used primarily or solely as an annealing separator. Yet it has been noted that a strip of silicon steel which has been decarburized and has had silica developed on and immediately within its surfaces, when coated on one side only with magnesia as herein taught, wound into a coil and annealed at about 2150 F. for 24 hours in dry hydrogen, exhibited a glass film of the type described in the Carpenter et al. patent above noted on both of its sides.

Modifications may be made in the invention without departing from the spirit thereof; and the method herein taught may be practiced with wide variations of apparatus. Having described the invention in certain exemplary embodiments, what is claimed as new and desired to be secured by Letters Patent is:

l. A process of forming on the surface of metallic sheet or strip stock a coating of annealing separator in dry form and devoid of water of constitution which coating is adherent to the said surface, which comprises creating between the said surface and at least one electrode spaced therefrom an electrostatic field by applying to said electrode an electrical potential sufiiciently positive with respect to the potential of said surface to produce a positive corona about the said electrode without producing a negative corona at the said surface, and introducing into the space between the said electrode and the said surface an annealing separator in finely divided form entrained in a dry, non-reactive gas, the position of introduction being closer to the said surface than to the said electrode and outside the area of the said corona.

2. The process claimed in claim 1 wherein the said annealing separator as introduced in the said gas is isolated from said electrode by a blanket of moving dry gas which is devoid of separator.

3. The process claimed in claim 2 wherein the sa1d surface is moving as so treated in a given direction with respect to said electrode, and in which said gases are moving in the same direction.

4. The process claimed in claim 3 wherein the quantity of gas in which said separator is entrained is such as to provide upon the said surface a coating of said separator in a thickness suitable for annealing use but not exceeding substantially 0.1 oz. per square foot of surface to be coated.

5. The process claimed in claim 3 wherein the quantity of gas in which said separator is entrained is such as to provide upon the said surface a coating of said separator in a thickness suitable for annealing use but not exceeding substantially 0.1 oz. per square foot of surface to be coated, in which the said electrode and the portion of said surface being coated are enclosed in a housing through which said gases pass, the velocity of said gases being controlled to avoid such turbulence within said housing as would bring portions of said separator into the vicinity of the said electrode.

6. A process of coating dry silicon-iron strip with dry magnesium oxide substantially free of water of constitution, and so as to form an adherent coating of said magnesium oxide on said strip, which comprises passing said strip through a housing, entraining metered quantities of magnesium oxide in a dry gas, and passing the said gas into the said housing at one end thereof adjacent the surface of said strip, creating between the surface of said strip and a plurality of electrodes located within said housing and spaced from each other and from said strip, an electrostatic field by applying to said electrodes a direct current potential sufficiently positive with respect to the potential of said strip to provide a positive corona surrounding said electrodes, but of insufiicient value to provide a negative corona at the said strip, and isolating the said gas bearing said magnesium oxide from said electrodes by a blanket of magnesium oxide-free gas mov ing therewith, whereby the said magnesium oxide becomes positively charged and is propelled by the said electrostatic field to the surface of said strip. where it loses its said charge and clings to the said surface and to itself by molecular attraction, whereby to build up a coating on the said strip of said magnesium oxide not exceeding substantially 0.1 oz. per square foot.

7. The process claimed in claim 6 wherein the said gases issue with the said strip from an opposite end of said housing.

8. In apparatus for the purpose described, an insulative housing, means for passing ferrous metal of sheet thickness through the said housing, electrodes in said housing spaced from the surface of said metal, means for entraining an annealing separator in a dry gas, means for introducing the gas into the said housing at a position adjacent the surface of said metal and relatively remote from the said electrodes, and means for applying to the said electrodes a direct current electrical potential sufliciently positive with respect to the potentialof said metal to form about the said electrodes a positive corona of limited extent without producing a negative corona at the surface of the said metal and without producing an arc adischarge between the said electrodes and the said met 9. The apparatus claimed in claim 8 including means for introducing into said housing in a position to isolate the stream of gas bearing said separator from said electrodes a blanket of gas devoid of separator.

10. The structure claimed in claim 9 in which the said electrodes comprise members of rod-like nature extending transversely to the length of the said metal, said electrodes being less in length than the width of the said metal, and being spaced from the said metal by a distance which does not exceed about one-half the distagce of the separation of the said electrodes from each at er.

11. The structure claimed in claim 10 wherein said electrodes include an electrode in the form of a plate separated from the surface of said metal and charged to a positive potential about one-half the value of the potential applied to the said other electrodes.

12. The structure claimed in claim 11 in which said metal is caused to move through said housing and said gases are caused to move therethrough in the same directron, and including means for controlling the velocity of the sa1d gases to minimize turbulence while permitting of separator upon the said References Cited in the file of this patent UNITED STATES PATENTS 2,217,444 Hill Oct. 8, 1940 2,221,338 Wintermute Nov. 12, 1940 2,334,648 Ransburg et al Nov. 16, 1943 2,352,252 Canetta a June 27, 1944 2,421,787 Helmuth June 10, 1947 3,466,906 Miller Apr. 12, 1949 2,796,845 Rendel June 25. 1957 2,806,803 Thackara et a1 Sept. 17, 1957 2,811,135 Hayford et al Oct. 29, 1957 FOREIGN PATENTS 503,112 Canada May 23, 1954 

1. A PROCESS OF FORMING ON THE SURFACE OF METALLIC SHEET OR STRIP STOCK A COATING OF ANNEALING SEPARATOR IN DRY FORM AND DEVOID OF WATER OF CONSTITUTION WHICH COATING IS ADHERENT TO THE SAID SURFACE, WHICH COMPRISES CREATING BETWEEN THE SAID SURFACE AND AT LEAST ONE ELECTRODE SPACED THEREFROM AN ELECTROSTATIC FIELD BY APPLYING TO SAID ELECTRODE AN ELECTRICAL POTENTIAL SUFFICIENTLY POSITIVE WITH RESPECT TO THE POTENTIAL OF SAID SURFACE TO PRODUCE A POSITIVE CORONA ABOUT THE SAID ELECTRODE WITHOUT PRODUCING A NEGATIVE CORONA AT THE SAID SURFACE, AND INTRODUCING INTO THE SPACE BETWEEN THE SAID ELECTRODE AND THE SAID SURFACE AN ANNEALING SEPARATOR IN FINELY DIVIDED FORM ENTRAINED IN A DRY, NON-REACTIVE GAS, THE POSITION OF INTRODUCTION BEING CLOSER TO THE SAID SURFACE THAN TO THE SAID ELECTRODE AND OUTSIDE THE AREA OF THE SAID CORONA. 